XI  B  RAR.Y 

OF  THE 

UNIVERSITY 
OF    ILLINOIS 


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AGRICULTURE 


NON  CIRCULATING 

CHECK  FOR  UNBOUND 
CIRCULATING  COPY 


UNIVERSITY  OF  ILLINOIS 

Agricultural  Experiment  Station 


BULLETIN  No.  306 


EXPERIMENTS  IN  CROSSING  VARIETIES 
AS  A  MEANS  OF  IMPROVING  PRO- 
DUCTIVENESS IN  CORN 


BY  L.  H.  SMITH  AND  A.  M.  BRUNSON 


URBANA,  ILLINOIS,  MAY,  1928 


SUMMARY 

Among  the  various  methods  that  have  been  proposed  for  increas- 
ing the  yield  of  corn  is  that  in  which  use  is  made  of  first-generation 
hybrid  seed  produced  by  the  simple  crossing  of  one  variety  with  an- 
other. 

Numerous  experiments  by  different  investigators  to  test  this 
method  have  given  conflicting  results.  Many  instances  are  recorded 
in  which  the  yield  of  the  cross  exceeded  that  of  the  higher-yielding 
parental  variety,  but  these  are  offset  by  numerous  other  cases  in  which 
no  advantage  from  crossing  was  obtained,  the  cross  often  proving  in- 
ferior to  either  of  the  participating  parents.  The  present  bulletin 
places  on  record  some  additional  experiments  in  which  are  involved 
different  types  of  hybrid  combinations  that  vary  in  their  response  to 
crossing. 

Crosses  between  common  varieties  of  ordinary  dent  corn  gave 
little  or  no  increase  in  yield  over  that  of  the  higher-yielding  parent. 
On  the  other  hand,  crosses  between  highly  selected  strains,  which  as  a 
consequence  of  intense  selection  for  special  characters  had  suffered 
inbreeding,  gave  a  marked  increase  in  the  yield  of  the  cross  over  that 
of  either  parent. 

The  indication,  therefore,  is  that  our  ordinary  varieties  of  corn, 
as  they  are  usually  propagated  on  the  farm,  do  not  undergo  inbreeding 
to  such  an  extent  as  to  benefit  by  simple  crossing. 

These  results  fail  to  support  the  recommendation  advanced  from 
time  to  time  to  cross  varieties  of  corn  as  a  simple  and  reliable  method 
of  increasing  the  yield.  On  the  contrary  the  results  indicate  that  the 
chances  for  a  loss  by  this  practice  are  much  greater  than  for  a  gain, 
especially  if  the  expense  in  time  and  effort  in  making  the  crosses  be 
taken  into  consideration. 

It  should  perhaps  be  pointed  out  again  that  these  experiments 
have  to  do  only  with  direct  varietal  crosses;  they  do  not  deal  with 
that  more  complex,  tho  highly  promising,  plan  of  corn  improvement 
which  involves  the  production  and  the  subsequent  crossing  of  self- 
fertilized  lines. 


EXPERIMENTS  IN  CROSSING  VARIETIES 

AS  A  MEANS  OF  IMPROVING  PRO- 

DUCTIVENESS IN  CORN 


BY  L.  H.  SMITH  AND  A.  M. 

One  of  the  methods  that  has  been  proposed  for  increasing  the 
yield  of  corn  is  the  application  of  what  seems  to  be  a  rather  generally 
prevailing  principle  in  nature  known  as  "heterosis,"  or  hybrid  vigor. 
It  is  a  well-known  fact  that  many  kinds  of  plants  as  well  as  animals 
when  mated  in  hybridization  of  various  degrees  exhibit  in  their 
progeny  of  the  first  generation  an  increase  in  vigor  manifested  by 
increase  in  size  or  by  acceleration  in  growth. 

The  application  of  this  principle  to  corn  production,  in  its  simpler 
aspect,  involves  the  use  of  first-generation  seed  produced  by  crossing 
two  different  varieties.  The  cross  must  be  made  anew  each  year  from 
the  original  parental  stock  in  order  to  furnish  first-generation  hybrid 
seed.  A  more  recent  development  of  the  idea  involves  preliminary 
selection  within  self-fertilized  lines  for  a  number  of  generations.  The 
present  investigation,  however,  deals  with  simple  varietal  crosses  and 
is  not  concerned  with  self-fertilized  lines. 

The  idea  of  crossing  varieties  of  corn  for  improvement  in  yield 
is  by  no  means  a  new  one,  for  such  a  plan  was  proposed  more  than 
fifty  years  ago  by  Professor  W.  J.  Beal,  working  at  the  Michigan  Ag- 
ricultural College.  Since  that  time  a  considerable  number  of  inves- 
tigators have  become  interested  in  this  problem  and  have  from  time 
to  time  reported  results  of  their  experiments  so  that  rather  extensive 
literature  on  the  subject  has  accumulated. 

It  is  of  interest  to  note  that  the  Illinois  Experiment  Station  fur- 
nished some  of  the  earlier  contributions  to  this  subject.  In  1892  it 
published  a  bulletin  by  G.  W.  McCluer  on  "Corn  Crossing,"  and  in 
the  same  year  issued  another  bulletin  entitled  "Experiments  with 
Corn,"  which  includes  an  account  of  experiments  along  this  line  by 
Morrow  and  Gardner. 

McCluer  in  his  experiments  crossed  such  diverse  types  as  sweet, 
soft,  pop,  and  dent  varieties.  The  weight  of  a  10-ear  representative 
sample  of  each  cross  is  compared  with  that  of  the  corresponding  paren- 
tal varieties.  Summarizing  the  results  it  is  to  be  observed  that  only 
four  out  of  eighteen  tests  indicate  any  increase  in  yield  over  the  higher- 
yielding  parent  due  to  crossing. 

Morrow  and  Gardner  in  their  first  experiment  tested  five  dif- 
ferent crosses  between  dent  varieties  and  in  three  cases  out  of  the 


JL.  H.  Smith,  Chief  in  charge  of  Publications  of  Soil  Survey  (formerly 
Chief  in  Plant  Breeding),  and  A.  M.  Brunson,  formerly  Associate  in  Plant  Breed- 
ing. 

375 


376  BULLETIN  No.  306  [May, 

five  obtained  an  increase  in  yield  over  the  higher-yielding  parent. 
Repeating  the  experiment  the  following  year,  they  found  that  the  cross 
exceeded  the  better  parent  in  two  trials  out  of  four.  Thus,  so  far  as 
these  Illinois  experiments  are  concerned,  the  question  of  the  use  of 
crossbred  seed  was  left  quite  unsettled. 

It  is  not  the  purpose  of  this  paper  to  go  further  into  the  history 
of  this  problem  nor  to  discuss  in  detail  results  of  the  rather  long  series 
of  investigations  by  different  workers  at  other  institutions,  for  such 
an  effort  would  be  only  to  repeat  what  has  been  excellently  done  by 
several  writers.1 

As  a  general  statement  covering  all  the  different  investigations  on 
record,  it  may  be  said  that  many  of  the  experiments  indicate  consider- 
able advantage  from  crossing,' comparing  the  yield  of  the  cross  with 
that  of  the  better  parent.  On  the  other  hand,  these  favorable  results 
are  offset  by  about  an  equal  number  of  unfavorable  results  wherein 
the  yield  of  the  cross  falls  between  that  of  the  parent  varieties  or  even 
below  the  yield  of  the  poorer  parent. 

On  the  basis  of  the  more  favorable  results  some  rather  extravagant 
claims  have  been  made  which,  if  not  sufficiently  qualified,  would  lead 
farmers  to  infer  that  this  method  of  crossing  two  varieties  of  corn 
offers  a  simple  and  sure  means  of  increasing  their  yields. 

It  was  to  obtain  further  evidence  upon  the  problem  of  hybrid 
vigor  in  general,  and  upon  the  effect  of  crossing  the  standard  varieties 
of  Illinois  in  particular,  that  the  present  investigation  was  undertaken. 
In  addition  to  the  ordinary  varieties  some  rather  unusual  material  for 
comparative  hybridization  studies  was  available  in  the  form  of  the 
well-known  "Illinois"  strains  selected  at  the  Illinois  Experiment  Sta- 
tion for  special  chemical  and  physical  characteristics  by  the  ear-to- 
row  method. 

FOUR  GROUPS  OF  CROSSES  TESTED 

The  cross  combinations  tested  in  these  experiments  fall  into  four 
categories,  as  follows,  with  respect  to  the  classes  of  parental  stocks 
employed : 

Group  A  includes  a  series  of  crosses  produced  between  well-known 
commercial  varieties.  All  of  those  which  at  one  time  were  considered 
"standard  varieties"  for  Illinois  are  included.  In  this  group  Reid 
Yellow  Dent  served  in  all  cases  as  the  pollen  parent. 

Group  B  includes  only  two  crosses,  one  between  a  variety  of 
sweet  corn  and  Reid  Yellow  Dent,  and  one  between  a  pop  corn  and 
Reid  Yellow  Dent,  the  latter  being  used  as  the  pollen  parent  in  both 
cases. 


JFor  the  reader  who  may  be  interested  in  tracing  the  development  of  this 
subject  reference  may  be  made  to  two  reviews  appearing  in  Volume  14  of  the 
Journal  of  the  American  Society  of  Agronomy,  one  by  F.  D.  Richey  and  the 
other  by  Fred  Griffee. 


1928]  EXPERIMENTS  IN  CROSSING  VARIETIES  OF  CORN  377 

Group  C  is  a  series  made  up  of  crosses  between  strains  produced 
at  this  Station  by  continuous  ear-row  selection  for  certain  special 
characters  including  high  protein,  low  protein,  high  oil,  low  oil,  high 
ears,  and  low  ears.  These  strains  were  crossed  one  with  another  in  the 
seven  different  combinations  listed  in  Tables  1  and  4. 

Group  D  is  made  up  of  the  same  special  strains  mentioned  in 
Group  C,  but  instead  of  being  mated  with  one  another  each  of  these 
strains  is  here  combined  with  Reid  Yellow  Dent  as  the  pollen  parent. 

The  crossbred  seed  was  produced  by  interplanting  in  a  field  of 
the  male  or  staminate  variety  individual  rows  of  the  respective  female 
or  pistillate  varieties.  By  detasseling  all  plants  of  the  latter,  fertiliza- 
tion took  place  only  thru  pollen  from  the  male  variety.  The  cross- 
bred seed  thus  produced  each  year  was  planted  the  following  year  in  a 
variety  test  along  with  seed  of  the  corresponding  parental  varieties. 

With  the  exception  of  the  sweet  and  pop  varieties,  which  were 
purchased  in  the  local  market,  care  was  taken  to  obtain  seed  of  the 
respective  varieties  from  the  same  sources  every  year,  usually  from 
growers  who  had  given  special  attention  to  the  propagation  of  a  par- 
ticular variety. 

In  the  variety  test  plots  effort  was  made  .to  secure  as  nearly  a 
perfect  stand  as  possible  and  no  attempt  has  been  made  in  reporting 
results  to  correct  the  yields  for  variation  in  stand.  As  a  matter  of 
fact,  however,  the  plots  were  usually  fairly  uniform  in  this  respect. 

The  plots  were  planted  in  duplicate  each  year  and  the  tests  ran 
from  three  to  five  years,  thus  providing  from  6  to  10  trials  for  each 
variety.  At  regular  intervals  a  plot  of  Reid  Yellow  Dent  was  planted 
to  serve  as  a  check.  The  hybrid  plots  were  planted  between  plots  of 
the  two  respective  parent  varieties  in  order  to  reduce  soil  variation 
within  any  given  comparison  to  a  minimum. 

RESULTS  OF  TESTS 

The  records  of  the  variety  trials  are  shown  in  Table  1,  which 
gives  the  annual  yields  of  the  duplicate  plots  for  the  parental  varieties 
alongside  the  corresponding  crosses.  There  is  also  given  the  average 
gain  (or  loss)  in  yield  of  the  cross  over  that  of  the  yield  of  the  higher- 
producing  parent. 

For  convenience  in  considering  the  results,  the  data  of  Table  1 
are  summarized  in  Tables  2,  3,  4,  and  5  according  to  the  four  groups 
of  crosses  described  above.  These  tables  show  the  gain  or  loss  as  a 
result  of  crossing,  basing  the  comparisons  upon  the  yield  of  the  higher- 
yielding  parent.  This  gain  or  loss  is  expressed  in  terms  of  bushels 
per  acre  and  also  in  percentage.  The  figures  in  the  last  column  of  the 
tables  indicate  the  statistical  significance  of  the  results  as  determined 
by  "Student's"  method. 


378 


BULLETIN  No.  306 


[May, 


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EXPERIMENTS  IN  CROSSING  VARIETIES  OF  CORN 


381 


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382 


BULLETIN  No.  306 


[May, 


It  is  obvious  that  for  practical  purposes  the  corn  grower  is  inter- 
ested in  any  possible  gain  to  be  obtained  over  the  yield  of  the  better 
parent,  rather  than  over  the  parental  average,  which  has  frequently 
been  considered  for  comparison  in  reports  of  similar  investigations. 
For  this  reason  the  yield  of  the  higher-producing  parent  entering  into 
the  cross  is  used  as  the  basis  for  comparison  thruout  this  discussion. 

Group  A — Common  Dent  Varieties  Crossed 
Table  2  shows  the  average  increase  or  decrease  as  a  result  of  the 
crossing  for  the  combinations  in  Group  A.  Since  this  group  is  made  up 

TABLE  2. — GROUP  A:  DENT  VARIETIES  CROSSED  WITH  REID  YELLOW  DENT 
(Summary  showing  gain  or  loss  based  upon  yield  of  higher-yielding  parent) 


G 

tin 

L 

DSS 

Odds  that 

X 

Reid  Yellow  Dent 

Bushels 
per  acre 

Percent 

Bushels 
per  acre 

Percent 

difference 
is  statisti- 
cally sig- 
nificant 

Champion  White  Pearl  . 
Boone  County  White.  .  . 
Learning  (Chester's)  .... 
Calico  

1,89 
1.61 
1.58 
.38 

3.91 
3.34 
3.67 
.91 

...     3 

6.0:1 
„  11.6:1 

*  32-  :l 
1.6:1 

Silvermine  

.18 

.38 

1.2:1 

Crimson  

.42 

.98 

1.4:1 

Reid  Yellow  Dent  
Golden  Eagle  

.53 

.84 

1.20 
1.79 

1.4:1 
2  0:1 

Riley  's  Favorite  
Learning  (Maxey's)  .... 

1.73 
3.93 

4.13 

8.94 

5.7:1 
31.9:1 

of  crosses  between  common  dent  varieties,  this  set  of  results  is  of  par- 
ticular interest  from  the  farmer's  standpoint. 

Ten  combinations  are  included  in  these  tests,  Reid  Yellow  Dent 
serving  as  the  pollen  parent  in  all  cases.  Crossing  appears  to  have 
produced  little  effect  upon  the  yields.  In  analyzing  the  data  it  is  ob- 
served that  the  figures  for  the  different  varieties  range  from  a  small 
gain  of  less  than  2  bushels  an  acre,  or  about  4  percent,  on  the  one 
hand,  to  a  loss  of  about  4  bushels  an  acre,  or  nearly  9  percent,  on  the 
other  hand.  These  differences,  in  the  main,  are  well  within  the  ex- 
perimental error.  According  to  the  column  of  "odds,"  the  single  case 
in  which  the  result  can  possibly  be  considered  significant,  and  that 
doubtfully  so  from  the  statistical  standpoint,  represents  about  a  9- 
percent  loss. 

Group  B — Sweet  and  Pop  Varieties  Crossed  With  Dent 
The  summarized  results  for  Group  B  are  given  in  Table  3.   This 
group  includes  only  two  combinations.    It  is  of  especial  interest,  how- 
ever, from  the  fact  that  the  parental  varieties  represent  widely  differ- 


1928} 


EXPERIMENTS  IN  CROSSING  VARIETIES  OF  CORN 


383 


ent  groups  of  maize.   In  one  case  sweet  corn  is  used  as  the  maternal 
variety,  and  in  the  other  pop  corn  is  the  female  parent. 

In  yielding  capacity  both  of  these  varieties  are  far  inferior  to  the 
dent  variety  which  is  used  here  as  the  pollen  parent.  The  average 
annual  yield  of  the  sweet  corn  was  only  16  bushels  an  acre  (Table  1), 
while  that  of  Reid  Yellow  Dent  was  44.7  bushels.  The  yield  of  the 
cross  between  the  two,  however,  was  only  a  fraction  of  a  bushel  less 
than  that  of  Reid  Yellow  Dent. 


TABLE  3. — GROUP  B:    SWEET  AND  POP  VARIETIES  CROSSED 

WITH  REID  YELLOW  DENT 
(Summary  showing  gain  or  loss  based  upon  yield  of  higher-yielding  parent) 


Variety 

G 

•tin 

L 

3SS 

Odds  that 

X 

Reid  Yellow  Dent 

Bushels 
per  acre 

Percent 

Bushels 
per  acre 

Percent 

difference 
is  statisti- 
cally sig- 
nificant 

Sweet  (Country  Gentle- 
man)   

.58 

1.30 

1.7:1 

Pop  (Rice)  

5.76 

13.50 

57    :l 

The  result  for  the  pop-corn  cross  was  not  quite  so  favorable  as 
that  for  the  sweet-corn  cross.  The  average  annual  yield  for  the  pop 
corn  was  19.6  bushels  an  acre,  while  the  corresponding  Reid  Yellow 
Dent  plots  gave  42.7  bushels.  The  cross  between  the  two  produced  an 
average  yield  of  36.9  bushels,  or  5.8  bushels  less  than  the  yield  of  the 
more  productive  parent. 

Group  C — Closely  Selected  Strains  Intercrossed 

Group  C  differs  from  the  preceding  groups  in  that  the  crosses,  in- 
stead of  being  between  ordinary  varieties,  are  between  strains  that 
have  been  produced  under  rather  intense  selection  by  the  ear-row 
breeding-plot  method  for  certain  special  characters.  These  include 
the  four  chemical  strains  selected  at  this  Station  for  high  protein,  low 
protein,  high  oil,  and  low  oil,  respectively,  since  1896.  At  the  time 
this  crossing  experiment  was  started  these  strains  had  been  under  this 
close  selection  for  sixteen  years,  and  the  two  strains  selected  for  high 
and  low  ears  respectively  had  been  under  similar  rigid  breeding-plot 
control  for  nine  years.  It  is  inevitable,  therefore,  that  more  or  less 
inbreeding  must  have  taken  place  in  all  of  these  strains.  It  is  a  well- 
established  fact  that  inbreeding  in  corn  is  accompanied  almost  uni- 
versally by  a  loss  of  vigor  and  a  consequent  decrease  in  yield.  It  is 
also  now  well  known  that  vigor  of  growth  lost  thru  inbreeding  is 
usually  restored  upon  crossbreeding. 

With  these  facts  in  mind  it  is  not  surprising,  therefore,  that  the 
results  in  Group  C  are  different  in  nature  from  those  of  the  preceding 


384 


BULLETIN  No.  306 


[May, 


groups.  Looking  over  the  results  in  Table  4,  no  loss  by  crossing  is 
observed.  On  the  contrary,  the  increase  in  yield  of  the  cross  over  that 
of  the  higher-yielding  parent  is  in  at  least  five  cases  out  of  the  seven 
of  a  very  substantial  order,  judging  from  the  magnitude  of  the  gains 
and  the  indications  of  the  odds. 

TABLE  4. — GROUP  C:    STRAINS  PRODUCED  UNDER  CONTINUOUS  SELECTION 

FOR  SPECIAL  CHARACTERS  INTERCROSSED 
(Summary  showing  gain  or  loss  based  upon  yield  of  higher-yielding  parent) 


Gf 

lin 

L( 

)SS 

Odds  that 

Crosses 

Bushels 
per  acre 

Percent 

Bushels 
per  acre 

Percent 

difference 
is  statisti- 
cally sig- 
nificant 

Low  Ear  X  High  Ear.  .  . 
High  Protein  X  Low  Oil 
Low  Oil  X  High  Oil.  .  .  . 
High  Oil  X  High  Pro- 
tein   

9.63 
8.75 
5.70 

4.76 

24.44 
25.76 
16.39 

13.49 

4999:1 
9999:1 
108:1 

1999:1 

High  Oil  X  Low  Pro- 
tein   

4  63 

10.95 

109:1 

Low  Oil  X  Low  Protein. 
Low  Protein  X  High 
Protein  

4.53 
3.44 

11.33 

8.36 

16:1 
17:1 

These  results  are  of  practical  interest  as  indicating  the  possibility 
of  restoring  the  yielding  capacity  of  varieties  or  strains  that  have  suf- 
fered deterioration  thru  inbreeding.  For  example,  a  means  is  suggested 
of  avoiding  the  detrimental  effect  of  reduced  yields  in  breeding  for, 
let  us  say,  high  oil.  If  two  independent  lines  of  high  oil  were  propa- 
gated, doubtless  the  loss  in  yield  accompanying  the  gain  in  oil  could 
be  remedied  by  combining  the  two  lines  or  strains  and  using  each  year 
seed  of  the  first-generation  cross. 

Group  D — Reid  Yellow  Dent  Crossed  With  Closely  Selected  Strains 
Each  of  the  special  strains  entering  into  the  crosses  of  Group  C 
was  crossed  with  Reid  Yellow  Dent  and  these  crosses  constitute 
Group  D,  the  results  for  which  are  given  in  Table  5.  This  set  of  tests 
was  conducted  for  only  three  years. 

The  results  for  this  series  of  crosses  are  not  very  decisive.  The 
figures  show  four  gains  for  the  cross  and  two  losses,  but  in  only  one 
case,  the  second  on  the  list,  do  the  odds  indicate  possible  statistical 
significance.  It  may  be  observed,  however,  from  data  in  Table  1,  that 
in  all  cases  the  average  yield  for  the  cross  is  above  the  average  yield 
for  the  selected  strain.  It  is  also  worthy  of  note  that  altho  in  all  cases 
excepting  one  the  "Illinois"  parent  was  lower  yielding  than  the  Reid 
parent,  the  hybrids  are  fully  as  high  yielding  on  the  average  as  those 
in  Group  A. 


1928} 


EXPERIMENTS  IN  CROSSING  VARIETIES  OF  CORN 


385 


TABLE  5. — GROUP  D:    STRAINS  PRODUCED  UNDER  CONTINUOUS  SELECTION 

FOR  SPECIAL  CHARACTERS  CROSSED  WITH  REID  YELLOW  DENT 
(Summary  showing  gain  or  loss  based  upon  yield  of  higher-yielding  parent) 


Strains 

Gi 

iin 

L< 

>ss 

Odds  that 

X 

Reid  Yellow  Dent 

Bushels 
per  acre 

Percent 

Bushels 
per  acre 

Percent 

is  statisti- 
cally sig- 
nificant 

Low  Ear  

6  33 

14  07 

8  3:1 

Low  Protein  

5  25 

11  44 

38  1:1 

Low  Oil  

3  38 

7  13 

4  1:1 

High  Oil  

1  15 

2  57 

2  4:1 

High  Ears  

1  47 

3  10 

7  2:1 

High  Protein  

2.72 

5.50 

6.7:1 

Simple  Crossing  of  Varieties  Does  Not  Insure 
Increase  in  Yield 

In  studying  the  results  of  these  experiments  on  the  effect  of  cross- 
ing varieties  of  corn,  two  sets  of  conditions  are  to  be  considered — one 
in  which  the  parent  varieties  have  been  subjected  to  close  breeding  and 
one  in  which  the  participating  varieties  have  not  been  close-bred. 

It  is  well  established  that  varieties  or  strains  which  have  been 
under  continuous  selection  by  methods  resulting  in  some  degree  of  in- 
breeding, like  those  of  Group  C  in  these  experiments,  are  likely  to 
receive  a  beneficial  effect  by  crossing.  On  the  other  hand,  corn  that 
has  not  undergone  inbreeding  to  a  detrimental  extent  is  not  so  likely 
to  receive  a  benefit  from  crossing.  According  to  this  principle,  the 
common  commercial  varieties  of  dent  corn  as  ordinarily  propagated 
on  the  farm  are  probably  naturally  in  such  a  hybrid  state  that  arti- 
ficial crossing  is  ineffective  in  increasing  the  vigor  of  growth  or  raising 
their  productiveness. 

The  theory  has  been  advanced  that  the  increased  vigor  manifested 
in  the  crossing  of  varieties  is  due  to  the  assembling  in  the  hybrid  of 
certain  growth  factors  not  common  to  both  parents.  With  this  as- 
sumption it  would  follow  that  the  greater  the  diversity  of  character- 
istics in  the  combining  varieties,  the  greater  the  chance  for  increase  in 
yield  of  the  hybrid.  Some  results  of  the  various  investigations  on  rec- 
ord appear  to  bear  out  this  theory.  Conceding  the  correctness  of  such 
an  hypothesis,  the  results  here  reported  might  be  explained  upon  the 
theory  of  diversity  of  characteristics  in  the  combining  varieties.  Thus 
in  the  common  dent  varieties  used  in  these  experiments  the  combining 
parents  probably  do  not  differ  greatly  in  their  heritable  characters  or 
genetic  make-up,  while  in  the  highly  selected  strains  the  combining 
parents  carry  sets  of  genetic  factors  that  differ  considerably.  In  the 
light  of  these  considerations  it  would  be  too  much  to  conclude  that  no 
benefit  is  ever  to  be  expected  from  the  simple  crossing  of  varieties;  but. 


386  BULLETIN  No.  306 

as  the  experiments  show,  there  is  great  probability  that  the  particular 
combination  that  would  reward  the  breeder  for  his  trouble  will  be 
missed. 

The  results  of  the  experiments  presented  herein  appear  to  be  in 
line  with  theoretical  considerations  and  they  lead  to  the  conclusion 
that  the  simple  crossing  of  varieties  as  a  practice  does  not  insure  an 
increase  in  yield;  on  the  other  hand,  the  chances  in  crossing  ordinary 
varieties  are  greater  for  loss  than  for  gain,  especially  if  the  expense 
of  time  and  effort  in  producing  crossbred  seed  is  considered. 


UNIVERSITY  OF  ILLINOIS-URBANA 


